The disclosure generally relates to determining formation properties, and more particularly relates to methods and apparatuses for determining formation properties using non-directional electromagnetic measurements in high angle or horizontal wells.
This section is intended to introduce various aspects of the subject matter described and/or claimed below. This discussion is believed to be helpful in providing background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, not as admissions.
Logging may be used in wellbores to make, for example, formation evaluation measurements to infer properties of the formations surrounding the borehole and the fluids in the formations. Typical logging tools may include electromagnetic (resistivity) tools, nuclear tools, acoustic tools, and nuclear magnetic resonance (NMR) tools, though various other types of tools for evaluating formation properties (also referred to as “formation parameters”) are also available. Early logging tools were run into a wellbore on a wireline cable after the wellbore had been drilled. Modern versions of such wireline tools may still be used extensively. However, as the demand for information while drilling a borehole continued to increase, measurement-while-drilling (MWD) tools and logging-while-drilling (LWD) tools have since been developed. MWD tools may typically provide drilling parameter information such as weight on the bit, torque, temperature, pressure, direction, and inclination. LWD tools may typically provide formation evaluation measurements such as resistivity, porosity, NMR distributions, and so forth. MWD and LWD tools may have characteristics common to wireline tools (e.g., transmitting and receiving antennas, sensors, etc.), but may be designed and constructed to endure and operate in the harsh environment of drilling.
Additionally, electromagnetic measurements may be used in downhole applications, such as logging-while-drilling (LWD) and wireline logging applications. For example, electromagnetic measurements may be used to determine a subterranean formation resistivity (including horizontal resistivity (Rh) and vertical resistivity (Rv)), formation dip, azimuth, as well as detection of bed boundaries. Further, sometimes alone or in conjunction with other formation measurements (such as porosity), electromagnetic measurements may be used to indicate the presence of hydrocarbons in the formation.
Non-directional tools may refer to tools that use antennas having magnetic dipoles that are parallel with the tool axis (sometimes referred to as a z-direction), and may be referred to as axial antennas. Non-directional measurements may, in certain instances, be referred to as “conventional” electromagnetic measurements. In low angle and vertical wells, non-directional resistivity measurements may be sensitive to Rh, with no and/or slight sensitivity to Rv. However, in high angle and horizontal wells, non-directional electromagnetic measurements may be sensitive to Rh, Rv, and formation dip. Moreover, in a homogenous formation, Rv and dip may be coupled, meaning that different pairs of Rv and dip values can produce the same z-z coupling response for a given axial transmitter and axial receiver pair. In such circumstances, Rv and dip may be difficult to distinguish from non-directional resistivity measurements alone in high angle or horizontal wells.
More recently, directional resistivity tools have been developed that may make use of tilted or transverse antennas (antennas that have a magnetic dipole that is tilted or transverse with respect to the tool axis). A transverse antenna may generate a radiation pattern that may be equivalent to a dipole that is perpendicular to the tool axis (by convention the x- or y-direction). A tilted antenna may be an antenna whose dipole moment is neither parallel nor perpendicular to the longitudinal axis of the tool. Tilted antennas may generate a mixed mode radiation pattern (i.e., a radiation pattern in which the dipole moment is neither parallel nor perpendicular with the tool axis). Electromagnetic measurements made by transverse or tilted antennas may be referred to as directional measurements. Such a directional arrangement (a tilted and/or transverse antenna) may produce a preferential sensitivity on one azimuthal side of the logging tool, which may enable the tool to better detect bed boundaries and other features of the subterranean formations to be identified and located. As such, when compared to conventional/non-directional resistivity measurements, directional resistivity responses may be better suited to determining formation characteristics in high angle or horizontal wells.
While some electromagnetic logging tools may be capable of making both directional and non-directional measurements, sometimes a particular drilling application or job may be limited to an electromagnetic logging without directional measurement capability. For instance, depending on various factors, such as cost constraints, some drilling jobs may be limited to conventional electromagnetic measurements. However, while such measurements are generally suitable for determining formation properties (such as Rh, Rv, and dip) in low angle or vertical wells, the determination of such formation properties can become increasing difficult in higher angle or horizontal wells.